If you look at the master cylinder in the shop mNUAL, there is a small hole at the bottom of the fluid reservoir - this hole allows pressure in the system to bleed fluid back in to the reservoir as the brake fluid heats up.

This hole is just about in the middle of the reservoir.

If the master cylinder has been changed or worked on, this hole should be checked by putting something (like a paper clip) in the hole to see how deep it goes.

If it only goes in a slight amount - like 1/8" or so, the piston in the master cylinder is in the incorrect position, blocking the bleed hole.

You may have to empty the reservoir to see this hole.

As the other poster mentioned, the push rod may have to be adjusted to correct this problem.

BTW - some sort of bleed is present in most older master cylinders - My 54 Chevy had the same problem after a mechanic changed my master cylinder - in the case of my car, a special tool had to be used to set it correctly.